The present invention relates to a photometer for measuring photometric quantity. More specifically, the present invention pertains to a photometer which is calibrated using a standard light source.
In general, a photometer has to be calibrated by reading out a photometric quantity (such as luminous flux, luminance, illuminance and so on) using a standard light source, and, where it is provided with a plurality of measurement ranges, these ranges have to be brought into proper correlation. Namely, the measurement values have to be made constant among all measured areas of an object with respect to the same photometric quantity, so as to precisely measure the photometric quantity. This correlation is accomplished by changing the measurement range and the measured area, which is done by changing density filters and the size of an aperture located at an imaging position of the objective, respectively.
Conventional photometers such as disclosed in U.S. Pat. No. 3,818,198 are so constructed that a photodetecting circuit includes a main impedance connected between a photoelectric element and an amplifier and a plurality of variable impedances connected with the main impedance. The number of the variable impedances is the same as the number of adjustment elements provided for adjusting the output of the photodetecting circuit. In such a photometer, the calibration and correlation are carried out by connecting a variable impedance of appropriate value with the main impedance.
It should, however, be noted that the above-mentioned photometer is very expensive because a plurality of variable impedances and means for connecting them with other parts must be provided. Also, the procedures for calibration and correlation are complicated, and it is difficult to precisely measure the photometric quality because the correlation must be made using a standard light source and an appropriate impedance among the variable impedances must be added to the main impedance at each measurement.
It is therefore an object of the present invention to provide a photometer which enables calibration and correlation without using any variable impedance.
Another object of the present invention is to provide a photometer in which the calibration and correlation can be carried out without following any specific procedure, so that it is possible to avoid degradation of measurement precision brought about by a change in the standard light source or vibration of the photometer.